Molded articles, and particularly blow molded structures such as bottles are commonly formed from polymers of 1-olefins such as polyethylene. It is important to the commercial utilization of a given polymer system that the converted product such as a bottle exhibit an optimized balance of properties, including for example, acceptable stress crack resistance and flexural stiffness. In addition, and in a contributing sense, it is necessary that the polymer exhibit suitable processability, i.e., satisfactory rheological behavior under flow and deformation during fabrication. Although the viscoelastic behavior of polymer melts has been the subject of considerable study, it has not proven possible to translate performances during fabrication to end use articles in such manner as to selectively determine polymerization and particularly catalyst requirements. Moreover, as in any case catalyst performance must also be measured in terms of efficiency or productivity and stability over a sensible life.
The use of chromium compounds in the polymerization of olefins is well-known. U.S. Pat. Nos. 2,825,721 and 2,951,816 teach the use of CrO.sub.3 supported on an inorganic material such as silica, alumina or combinations of silica and alumina and activated by heating at elevated temperatures to polymerize olefins. When these catalyst systems are used in various polymerization processes such as the well-known particle-form process, the resins produced, while useful in many applications, are unsatisfactory for others because of a deficiency in certain properties such as melt index.
Improved chromium based supported catalysts are known, particularly those disclosed and claimed in U.S. Pat. Nos. 3,984,351 and 3,985,676. Such catalysts permit the production of resins of improved flow properties and shear response, but have been found difficult to employ on a commercial scale without product segregation or resin blending because of variation in rheological properties of polymer produced, relative to its use in fabrication and especially blow molding e.g. in accumulator, or accumulator ram equipment.
An examination of this phenomenon utilizing now classic measures of resin shear response (HLMI/MI values determined according to ASTM-D-1238, Conditions F/E) evidenced no apparent reason for differential performance of these resins in fabrication equipment. Empirical studies suggested that a more exacting viscosity analysis was required to isolate resin candidates adapted to afford shortened cycle times or otherwise improved performance in selected blow molding equipment. It has been found that such a determination may be made using a viscosity ratio of Eta.sub.1 /Eta.sub.1000, broadening the range covered and expressly including the range 1 to 1000 reciprocal seconds. These values provide a correlatable measure of critical performance in end use as more fully described hereinafter, and permit the selection of resin candidates particularly adapted to use in such blow molding equipment as the accumulator or accumulator ram equipment aforementioned.
Further studies of certain chromium catalysed resin variability in terms of this viscosity ratio characteristic permitted identification of production factors critical of the controlled production of resins of the desired characteristics.
Manyik et al., in U.S. Pat. Nos. 3,231,550 and 3,242,099 describe poly(hydrocarbylaluminum oxides) produced by the reaction of water with an organo hydrocarbylaluminum compound, which are in turn reacted with transition metal e.g., chromium compounds and used as olefin polymerization catalysts.
Shida et al. in U.S. Pat. No. 3,882,096 disclose ethylene catalysts comprising a support impregnated with chromium oxide and the reaction product of water and an alkyl ester of titanium, and heat activated.
Long in U.S. Pat. No. 3,152,105 shows an .alpha.-olefin catalyst comprising a carboxylic acid salt of chromium, water and an organoaluminum compound.
Hogan et al. in U.S. Pat. No. 3,288,767 disclose a process for the control of shear response in the preparation of polyolefins, by varying the water vapor content of the catalyst activation art.
Witt in U.S. Pat. No. 3,378,540 produces 1-olefin polymers of controlled melt index by utilizing a catalyst system formed from a hydrogel support of controlled water content.
Modifications in silica gel for catalytic activity are shown in Burwell, Chemtech, pp. 370-377 (1974) and Peri, J. Cat. 41, pp. 227-239 (1976). None of these prior art disclosures refer to the control of resin rheological characteristics in a process for the polymerization of 1-olefins utilizing a support, heat activated chromium catalyst.